In the past, metal plate was typically treated with chromate to form a chromate layer in order to improve the corrosion resistance of cold rolled steel plate, zinc-plated steel plate, zinc alloy-plated steel plate, aluminum-plated steel plate, and so forth, used in automobiles, home appliances and construction material applications. Electrolytic chromate and coated chromate are examples of this chromate treatment. In the case of electrolytic chromate, for example, sheet steel was treated by cathode electrolysis using a bath having as its main component chromic acid to which were also added sulfuric acid, phosphoric acid, boric acid and various type of halogen and other ions. In addition, in the case of coated chromate, due to the problem of elution of chromate from the chromate treated steel plate, a process is known, for example, in which the steel sheet is treated with a liquid containing inorganic colloids and inorganic anions in addition to chromic acid in which a portion of the hexavalent chromium was reduced to trivalent chromium, in advance, or to chromic acid having a specific ratio of hexavalent chromium to trivalent chromium. In addition, other methods that have been developed include a method in which the chromium is blocked by compounding with an organic polymer, and a method in which the chromate layer is additionally covered with an organic polymer.
Although chromate layer formed by electrolysis exhibit a low level of elution of hexavalent chromium, their corrosion resistance cannot be said to be adequate. In addition, the chromate layer is susceptible to damage during machining. Thus, there are certain problems with corrosion resistance after machining. In addition, in the case of chromate layer formed by coating, when used without modification after treatment, elution of a portion of the hexavalent chromium from the chromate layer cannot be avoided. Resin-chromate has been developed to reduce this dissolving of the chromate layer. However, deterioration of the resin due to the high oxidation effects of chromic acid cannot be avoided, thus preventing these chromate layers from having adequate coating reliability in terms of industrial use. Although improved technologies can be found, such as that which makes various adjustments in the resin structure as disclosed in Japanese Unexamined Patent Publication No. 5-230666, and that which attempts to achieve significant improvements in workability and long-term coating stability involving the response of the chromate layer to a corrosive environment by regulating the form in which hexavalent chromium exists in the chromate layer as disclosed in Japanese Patent Application No. 7-149200, filed by the inventors of the present application, none of these can be said to be adequate from the viewpoint of completely inhibiting elution of hexavalent chromium.
In this way, in order to completely inhibit elution of hexavalent chromium, it is necessary to develop a rust-preventive layer having functions identical to chromate layer containing hexavalent chromium of the prior art, but without using any hexavalent chromium whatsoever.
Until now, corrosion inhibitors have been developed for the purpose of inhibiting corrosion of metal placed in a corrosive environment. This consisted mainly of adding a trace amount of an inhibitor to a corrosive solution, adsorbing onto the surface of the metal, and forming a passivating layer to decrease activation of the metal surface and inhibit ionized elution, and many such materials are known. Prominent examples of inorganic compounds of these materials include hexavalent chromium salts, silica, phosphates and vanadates. Prominent examples of organic compounds include carboxylic acids such as benzoates and azelates, and compounds containing --S-- and --N-- which easily form complexes with metal ions. However, since these compounds demonstrate effects when trace amounts are added to a corrosive solution, those compounds that are able to form a layer on a metal surface and clearly demonstrate long-term reliability are only chromate treatment and phosphate treatment including phosphate treatment. In the case of organic compounds in particular, their reliability when used as single layer is extremely low.
For example, a paint composition and layer have been proposed that are characterized by containing 0.01 to 10 percent by weight (as solid) of an organic corrosion inhibitor having a nitrogen atom in its molecule in a water-based paint as disclosed in Japanese Unexamined Patent Publication No. 4-318071 and Japanese Unexamined Patent Publication No. 5-214273. Although water-soluble organic corrosion inhibitors and slightly water-soluble organic corrosion inhibitors proposed in the above are both indicated in said patents, no clear distinction therebetween is made. In the case that the organic corrosion inhibitor is water-soluble, it elutes outside the layer when moisture enters in a corrosive environment, thereby preventing it from demonstrating adequate corrosion resistance. In addition, in the case of a slightly water-soluble organic corrosion inhibitor, it is typically extremely difficult to disperse the inhibitor in water-based paints and, if simply mixed, causes aggregation in the paint or in the layer formed. Since this impairs the uniformity of the paint or layer, the stability of the paint, as well as the resulting corrosion inhibitory effects, are inadequate. As a result of corrective measures still having not been taken despite the existence of these problems, the corrosion resistance that is obtained is inadequate.
In addition, in Japanese Unexamined Patent Opposition No. 7-97534, although a paint is proposed that contains 0.05 to 25 percent by weight of one or more types of alkynes, alkinols, amines or their salts, thio compounds, heterocyclic compounds, polycarboxylic acid compounds or their salts, aromatic carboxylic acid compounds or their salts and lignin sulfonates or their salts, during application to metal plate, the formation of a chromate layer or zinc phosphate layer that serves as a rust preventive layer is essential for pretreatment, and it cannot be expected to demonstrate corrosion resistance when used as a single organic layer.
As has been described above, in the case of an single layer containing an organic corrosion inhibitor, when the organic corrosion inhibitor is water-soluble, it elutes outside the layer when water enters in a corrosive environment, thereby preventing adequate corrosion resistance. In addition, when the organic corrosion inhibitor is hardly water-soluble, it aggregates in the paint or layer thereby creating problems in the dispersion method or dispersion form. It is assumed that this type of organic corrosion inhibitor is unable to demonstrate adequate corrosion resistance because a site at which the organic corrosion inhibitor is able to act efficiently cannot be obtained within the layer. Moreover, since the effect of organic corrosion inhibitors consists of inhibiting corrosion by forming a complex with metal ions, or in other words, anodic corrosion resistance which functions mainly by inhibition of metal ionization involving deposition at the interface after forming a complex with eluted metal ions, and the pH region at which complex-forming functional groups required for complex formation are dissociated is unevenly distributed primarily in the neutral region, at low pH regions of the anode portion or in a rising pH environment present during the early stages of corrosion, the effects of the organic corrosion inhibitor are expected to be weak. In addition, aside from their corrosion prevention ability, since the layer formation capability of these organic corrosion inhibitors onto a metal surface is generally inferior to inorganic corrosion inhibitors and is considerably dependent upon environmental changes at the interface, it results in the problem of difficulty in maintaining stable adhesiveness. As a result, an organic compound that offers both corrosion prevention and adherence to a certain extent must be selected, thus making further decreases in corrosion prevention unavoidable. The uses of these corrosion inhibitors are limited to certain types of metals, and nearly all are limited to use as paint additives.